Will give how can william carlos williams' poem, "this is just to say," and william shakespeare's sonnet 18, "shall i compare thee to a summer's day? " belong to the same broad genre called poetry?

You have to be very careful when dealing with "This is just to Say."

If you go off the deep end and destroy it's ordinariness by waxing over it, or strangle it by over interpretation, you will certain take away what is poetic about it.

I wouldn't go much deeper than saying what it seems to be implying on the surface.  

I'm going to take a chance and go just one step further. We quite often act selfishly even when we know it is wrong. We know things can be tempting and irresistible and we give into that impulse. Sorry just doesn't cover it, but he says it does.

The last paragraph before this one is all that can be said. You may come up with something else and that's fine, but you don't get to add more than 1 thought. It is poetry, even if you do have to dig somewhat. But dig gently.

Sonnet 18

No doubt about this one. It is poetry. The language and form are poetic. Think of it this way, especially if you are female (which has a 50% chance of being right): wouldn't you really like to hear someone say this to you? If you are male, wouldn't it give you delight to say it to someone?

The Thought is poetic. Shall I compare you to a summer's day. Shakespeare is going to have to work to pull that off. Of course, he's Shakespeare, so we don't doubt he can do it.

The Language is poetic. Pick any line at random. Try this: Nor lose possession of that fair thou owest.  We would say it as Nor (will you) lose possession of the fairness you own. Each line does that. It does not say anything much beyond what it says, but is it ever strong and powerful in the way it is said. Two poems two different styles. One very small and suggestive and delicate, and the other strong and masculine and powerful.

Both poems. Both strong in their method.

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which shakespeare reading? you failed to specify.